1. Field of the Invention
The present invention relates to apparatus and methods for providing a boom extension for gantry cranes having a retractable boom which in its operating position extends outboard from the crane superstructure in a horizontal orientation and which can be pivoted about its inboard end at its attachment point to the crane superstructure by boom hoist ropes to an upward projecting retracted position. More particularly, the present invention relates to a retrofit for existing cranes that does not require lowering the retractable boom to ground level to attach the boom extension to the outboard end of the retractable boom, nor does it require increasing the lift capacity of the boom hoist machinery to hoist the retractable boom and boom extension combination to its retracted position, nor does it require strengthening the retractable boom to carry a larger bending moment due to the increased loading created by the dead weight of the boom extension.
2. Description of the Prior Art
Rail mounted dockside gantry cranes having retractable booms have long been used, and are the standard in the industry, for loading and unloading containerized cargo and fungible bulk materials between waterborne vessels and dockside transportation equipment. The retractable boom in its operating position extends horizontally outboard from the crane superstructure and in its retracted position clears any ship berthed alongside the dock adjacent the crane. When a ship is berthed alongside a dock, the gantry crane is moved along the dock parallel the ship to a position where the retractable boom can be extended across the beam of a ship above the ship's cargo-carrying areas. The cargo can then be transported between a dockside deposition area and any storage position located within the ship's holds or on the ship's deck.
In the particular form of gantry crane to which the present invention pertains, the retractable boom is pivotable about its inboard end at its attachment point to the crane superstructure. Boom hoist ropes raise the boom to an upward projecting retracted position. This permits a ship to be moved forward or backward alongside the dock during berthing or departure without mechanical interference with the dockside crane when the boom is retracted. Likewise, it also permits the crane to be moved fore and aft along the ship with the boom retracted to reposition the boom to service the different cargo-carrying areas of the ship without the crane hitting the vessel's superstructure or rigging when the crane is moved.
As the sea-going vessels which carry containerized or fungible bulk cargos have become larger to satisfy the economics of international trade, their beam width has been increased to accommodate a greater number of containers or larger amounts of fungible bulk materials athwartships. To effectively and efficiently load and unload such vessels, it has become necessary to extend the operational outreach of the dockside cranes so the containers or bulk materials can be deposited or retrieved from said vessels at the furthest athwartship locations relative to the dock.
Many existing dockside gantry cranes do not have the required outreach for servicing these newer vessels but are capable of handling the increase in weight of a loaded boom with an extension. It therefore is obviously desirable to provide a means to effect an increase in the length of such present gantry crane booms rather than to replace the whole crane.
Extensions of gantry crane operational outreach have been accomplished by utilizing a boom whose length has been increased from a nominal 125 feet to a length of 155 feet. With such an extension, the weight of the boom will be increased by as much as 20 tons which generally requires an increase in the overall structural strength and rebalance of the total crane structure to safely support the increase in the crane's total dead weight due to extended boom length. It is necessary to counter the increased tipping torque created by the added weight of the boom extension and the increased distance the suspended loads can be moved outboard from the crane superstructure.
In most retrofit boom conversions, it has been necessary to lower the existing boom to ground level, extend its overall length by the conventional means of adding a boom extension section, increase the strength of the original boom to resist the increased bending moment imposed on the original boom by the extension, and to lift the modified boom and reattach it to the crane superstructure. Retractable booms are typically located approximately 80 feet or more above dock level and can weigh generally between 60 to 100 tons. Lowering such a boom from such a height requires a substantial lift capacity and high lift capability of auxiliary erection equipment which is not always available and invariably is very expensive.
In making such a conversion, it is also usually necessary to at least increase the boom hoist machinery lift capacity and the boom hoist rope lifting strength. The weight of the boom extension severely overloads the boom hoist apparatus especially as a result of the boom extension being added to the outboard end of the original boom.
In order to accomplish these retrofits, it is necessary to effect a complete shutdown of the operation of the crane for an extended period of time and to incur the expense of heavy-duty lift machinery and auxiliary erection equipment to lower and lift the large structural components to effect the conversion. A substantial period of down time on the order of two to three months or more is required to effect the modifications and return the crane to operating condition.
The present invention provides a way of avoiding the necessity of making the main expensive modifications, of greatly shortening the time required to make the conversion, and of substantially reducing the cost involved for heavy lift erection equipment.
The present invention is an apparatus and method for modifying presently existing gantry cranes to provide a boom extension without the necessity of lowering the retractable boom to ground level to add the new boom section; without increasing the strength of the retractable boom to carry additional bending moment; without increasing the lift capacity of the boom hoist rope machinery or the strength of the boom hoist ropes; and the conversion can be accomplished in far less time and with far less additional lift capacity than heretofore required by contemporary conversion means and methods.